Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Automotive transmissions generally offer at least one negative speed ratio for reverse movement. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
Many transmissions are designed to operate at a discrete number of fixed speed ratios. The available speed ratios may be referred to as gears or gear ratios even if there is not a direct correspondence between each ratio and a particular physical gear within the transmission. The ratios are typically numbered starting from the highest speed ratio and progressing to lower speed ratios. For example, 1st gear may have a speed ratio of 4.5, 2nd gear a speed ratio of 3.0, 3rd gear a speed ratio of 2.3, etc. A particular speed ratio from the set of available speed ratios may be selected by engaging particular shift elements such as clutches or brakes. Shift elements may include actively controlled devices and passively controlled devices like one way clutches. Typically, shift elements are engaged by routing pressurized fluid to corresponding clutch apply circuits in a transmission valve body.
Some vehicles, commonly called hybrid electric vehicles utilize one or more traction motors and electrical energy storage, such as a battery, to reduce fuel consumption. In a number of operating conditions, the engine can be shut off and all propulsion performed by the traction motor using energy stored in the battery. The battery is charged by recapturing energy during braking. Additionally, when the engine is running, it may produce more power than needed for current propulsion with the extra power stored in the battery. Since engines are typically more efficient when operating at higher power levels, this reduces total fuel consumption. In a conventional vehicle, a number of vehicle functions rely on continuous power from the engine. In a hybrid vehicle, since the engine may be off much of the time, these functions must be implemented differently. One such function is the provision of pressurized hydraulic fluid to engage transmission shift elements.